He had two glass tumblers, one filled with clean water, and the other with whiskey. He placed a live worm in each glass. The worm in water was perfectly fine, however the worm which was dropped in whiskey died almost instantly.
Rather pleased with the experiment, he decided to ask the class what they could deduce from it. The class were silent for several seconds, until one boy at the back called out: "Drink whiskey and you won't get worms!"